Band-pass filter with variable band width



June 6, 1939. T. J. WEYERS BAND-PASS FILTER WITH VARIABLE BAND WIDTH Filed June 4, 1937 INVENTOR THEODORUS J. WEYERS ATTORN EY Patented June 6, 1939 BAND-PASS FILTER WITH VARIABLE BAND WIDTH Theodorus J. Weyers, Eindhoven, Netherlands,

assignor to N. V. Philips Gloeilampenfabrieken, Eindhoven, Netherlands, a corporation of the Netherlands Application June 4, 1937, Serial No. 146,411 In the Netherlands June 29, 1936 7 Claims.

This invention relates to a radio receiving circuit arrangement comprising one or more bandpass filters with variable band width.

To modify the width of the incoming frequency band, radio receiving arrangements are used comprising one or more band-pass filters each consisting of two or more mutually coupled circuits tuned to the same frequency, the coupling betweenthe tuned circuits being variable. In this case the widthof the received frequency band can be altered in a simple manner by modifying the coupling. Band-pass filters with capacitative coupling have the drawback that a modification of the coupling causes a variation of the tuning. This defect can be obviated by using inductively coupled band-pass filters, the coupling being varied by altering the mutual distance of the inductance coils of the tuned circuits.

To obtain, for a large width of the received frequency band, as uniform as possible an amplification of all incoming frequencies, one or more band-pass filters are used whose circuits are either critically or less than critically coupled in combination with one or more hypercritically coupled band-pass filters. The resonance curve of a critically or subcritically coupled band-pass filter exhibits one maximum for a frequency corresponding to the natural frequency of the circuits of which the band-pass filter consists, whereas the resonance curve of a hypercritically coupled band-pass filter exhibits two maxima viz. for frequencies on either side of the natural frequency of the circuits, a minimum occurring for the natural frequency of the circuits. By a suitable combination of both kinds of band-pass filters an approximately rectangular total resonance curve and consequently a uniform amplification of all frequencies within the received frequency band can be achieved. Instead of a bandpass filter whose circuits are subcritically coupled a single tuned circuit may be used.

It has now been found that with a radio-receiving circuit arrangement comprising one or more band-pass filters with variable band width and each consisting of two or more inductively coupled circuits, which are hypercritically coupled at least for large values of the band width and which are tuned to the same frequency, while moreover one or more single tuned circuits and/or critically or subcritically coupled band-pass filters are provided, a certain. distortion occurs more particularly for large band widths. This distortion is due to the fact that the total resonance curve of such a receiving arrangement is unsymmetrical.

According to the invention this distortion is removed by choosing the natural frequency of the coupled circuits of the hypercritically coupled band-pass filter so that the middle of the transmitted frequency band exactly coincides with the received carrier frequency at least for maximum band width.

The invention is based on the realization that the said symmetry of the total resonance curve is due to the fact that the middle of the transmitted frequency band for a hypercritically coupled band-pass filter does not coincide with the natural frequency of the circuits of which the band-pass filter consists as can be proved by calculations. the coupled circuits it is possible to make the middle of the transmitted frequency band exactly coincide with the incoming carrier frequency.

Since the said 7 distortion mainly occurs for large values of the band width an appreciable improvement over prior arrangements can be achieved by giving the circuits of the hypercritically coupled band-pass filter such a fixed detuning relatively to the remaining circuits of the arrangement, that the middle of the transmitted frequency band exactly coincides with the carrier frequency to be received for the largest value of the band width.

Another improvement can be obtained by adding to at least one of the coupled circuits of the hypercritically coupled band-pass filter a variable impedance such as a variable condenser which is controlled in such a manner that the natural denser maybe so proportioned that for the largest band width the middle of the transmitted frequency band exactly coincides with the carrier frequency to be received. However, it may also be so designed as to be variable in such a .manner, that the middle of the transmitted frequency band exactly coincides with the carrier frequency for any value of the coupling.

This condenser may, for instance, consist of two condenser electrodes secured to the corresponding ends of the inductance coils of the two circuits so that upon varying the coupling by altering the mutual distance between the coils the capacity of the condenser is simultaneously modified in the desired manner.

The invention will be more clearly understood by reference to, the accompanying drawing wherein Fig. 1 illustrates one form of a bandpass filter according to the present invention. Fig. 2 is analternative form of filter section that may be used in place of the section marked II By altering the natural frequency of coupled circuits II and I2.

in Fig. 1, and Fig. 3 is still another filter section that may be used for the portion II of Fig. 1.

Referring now to Fig. 1 which shows by way of example the intermediate frequency part of a radio receiving arrangement, the intermediate frequencies are supplied by the first detector tube I to the firstcircuit of the band-pass filter I, whose coupling is variable, but remains always below the critical value or corresponds thereto at the most. The oscillations amplified by the tube 2 are supplied to the first circuit of the band-pass filter II whose coupling is also variable. However, the coupling of the band-pass filter II, at least for large values of the band Width, exceeds the critical value. The intermediate frequency oscillations are then supplied to the detector 1. According to the invention the variable condensers 5 and 6 are connected in parallel with the tuning condensers 3 and 4 of the band-pass filter II. The adjusting device of these condensers 5 and 6 is mechanically coupled with the adjusting device for altering the band width such as indicated by the dotted line 8. Upon altering the coupling between the coils 9 and III the capacity of the condensers 5 and 6 is also altered so that with any adjustment of the coupling the middle of the frequency band transmitted by the band-pass filter II coincides as Well as possible with the incoming carrier frequency to which the circuits of the band-pass filter I are tuned.

It is not necessary that variable condensers should be connected in parallel with both circuits of the band-pass filter. By connecting a variable condenser in parallel with only one of the circuits it can also be achieved that the middle of the transmitted frequency band coincides comparatively exactly with the incoming carrier frequency for any value of the band width.

Fig. 1 represents an intermediate frequency amplifier but it will be appreciated that the invention can also be used in the radio frequency amplifier.

Fig. 2 shows another manner of ensuring the desired variable detuning of a hypercritically coupled band-pass filter. The band-pass filter represented in Fig. 2 comprises two inductively The corresponding ends of the two coupled inductance coils are connected through a variable condenser l3. The adjusting device of the condenser I3 is mechanically coupled with the adjusting device for controlling the band width such as indicated by the dotted line l4. Upon altering the coupling between the inductance coils of the band-pass filter the capacity of the condenser I3 is also modified so that the middle of the frequency band transmitted by the band filter continues to coincide as Well as possible with the incoming carrier frequency to which are tuned the single oscillatory circuits or subcritically coupled bandpass filters present in the receiving arrangement.

One variant of the construction illustrated in Fig. 2 is schematically represented in Fig. 3 and comprises an inductively hypercritically coupled band-pass filter consisting of the tuned circuits l5, l6 and I1, I8. Alteration of the coupling is effected by shifting the inductance coils l6 and IT relatively to one another. Two condenser electrodes [9 and 20 are secured to the corresponding ends of the inductance coils l6 and I1. Upon varying the band Width the capacity of the condenser constituted by the electrodes 19 and 20 varies in the desired manner.

What I claim is:

1. A radio receiving circuit arrangement comprising a plurality of band-pass filters capable of transmitting a variable band width, one filter consisting of two inductively coupled circuits which are hypercritically coupled at least for large values of the band width and which are tuned to the same frequency, and another filter comprising a pair of tuned circuits constituting a critically coupled band-pass filter, characterized in that the natural frequency of the hypercritically coupled band-pass filter is so chosen that at least for maximum band width the middle of the frequency band transmitted thereby exactly coincides with the carrier frequency to be received.

2. A radio receiving arrangement as claimed in claim 1, in which at least one of the tuned circuits of the hypercritically coupled band-pass filter has added to it a variable impedance which is so controlled that the natural frequency of the circuit decreases with increasing width of the transmitted frequency band.

3. A radio receiving arrangement as claimed in claim 1, in which at least one of the tuned circuits of the hypercritically coupled band-pass filter has connected in shunt thereto a variable condenser which is so controlled that the nattural frequency of the circuit decreases with increasing width of the transmitted frequency band 4. A radio receiving arrangement as claimed in claim 1 in which at least one of tuned circuits of the hypercritically coupled band-pass filter has connected in shunt thereto a variable condenser which is so controlled that the natural frequency of the circuit decreases with increasing Width of the transmitted frequency band, and in which the shunt variable condenser and the variable coupling means of both pairs of coupled circuits are mechanically coupled for unicontrol operation for controlling the band width.

5. A radio receiving arrangement as claimed in claim 1, in which one of the ends of one of the circuits of a hypercritically coupled bandpass filter consisting of two circuits is connected to the corresponding end of the other circuit through a condenser which is so proportioned that at least for maximum band width the middle of the frequency band transmitted by the band-filter exactly coincides with the carrier frequency to be received.

6. A radio receiving arrangement as claimed in claim 1, in which the high potential ends of the hypercritically coupled circuits are connected through a condenser which is so proportioned that at least for maximum band width the middle of the frequency band transmitted coincides substantially with the received carrier frequency, said con-denser being constituted by two condenser electrodes secured to the corresponding ends of both inductance coils of the band-pass filter circuits.

7. A receiving circuit comprising a first pair variably coupled circuits, each fixedly tuned the same operating frequency, a second pair of variably coupled circuits also fixedly tuned to said operating frequency, means for varying the coupling of the first pair of circuits through a range from below hypercritical coupling to hypercritical coupling, reactive means connected in shunt to at least one of the second pair of circuits for varying its natural frequency, and means for operating all of the above mentioned variable means in unison.

THEODORUS J. WEYERS.

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